Sawing cords are increasingly used to saw blocks of stone into slabs for architectural or home interior use. The types of material sawn have steadily increased from the softer metamorphic stones such as marble to the extremely hard igneous rocks like granite (in all its shades of hardness and origin). In a saw machine using a sawing cord the cord is tensioned between at least two pulleys of which at least one is driven while the cord is pushed through a stone. ‘Sawing wire’ or ‘saw wire’ is many times used as an alternative term for ‘sawing cord’ but in this application the latter is preferred as it discriminates the product from other saw tools that are based on a single filament. A cord is known to be multi-filamentary.
While initially single loops of sawing cord were used to extract blocks out of quarries currently multi-loop slabbing machines have been designed driving up to 80 individual loops. Such loops are from 15 to 30 meters long and carry between 35 to 40 beads per meter so there are between 525 to 1200 beads per loop. Currently these beads are made via a powder metallurgy route (already described in U.S. Pat. No. 2,679,839 filed in 1952).
In this process an annular abrasive element is made from diamond grit that is thoroughly mixed with metal powder and an optional organic wax for forming a paste. The mixture of metal powder normally contains high melting temperature components such as cobalt, tungsten, iron, nickel sometimes in combination with low melting temperature components such as copper, tin, silver to improve consolidation. Possibly elements or compounds such as tungsten, phosphorous or tungsten carbide can be added to influence hardness and wear of the bead. The mixture is brought in a mould. This preform is sintered into a high density bead by application of temperature and possibly pressure (by ram pressing in the mould or by applying isostatic pressure through immersion in a high pressurised fluid). Suitable gasses are applied in order to prevent the powder from oxidising during sintering.
Producers will keep the combination of time and temperature below certain limits to prevent damage to the diamond particles. Diamond tends to graphitise (turn into graphite) when exposed to temperature above about 700° C. in air and above 1200° C. in an inert gas or vacuum. This graphitization starts from the outside of the diamond and results in a blackened, non-transparent diamond.
The resulting metallography of the sintered metal powder shows an amalgam of globular regions held together with low melting alloys.
After sintering the annular abrasive element is mounted on a metallic sleeve by means of a braze thus forming a sawing bead. In case of hot isostatic pressing annular abrasive elements are directly bonded to the sleeve and no brazing is needed. The beads are threaded on a steel cord and subsequently polymer is injection moulded in between the beads to keep them in position and to protect the steel cord from the environment.
Sawing cords have standardised around two sizes:                For quarrying and heavy duty work beads with an initial outer diameter of 9 to 11 mm are used. The steel cord has a diameter of 4.95 mm. The sleeves are 11 mm long and have an outer diameter of 7 mm.        For slabbing machines an initial outer bead size of 7.2 mm is standard. The steel cord has a diameter of 3.5˜3.6 mm. The outer diameter of the sleeve is 5.0 mm and is 11 mm long. The current benchmark for slabbing is that at least 10 square meter of material can be cut per meter of sawing cord on a Class A type of granite.        
There is a trend towards even smaller sizes: outer diameters of beads of 3.0, or even of 2.5 mm are being considered. Smaller bead sizes lead to more slabs out of a block per cut as less material is lost. Another trend is to have more but shorter beads per meter of sawing cord. More beads leads to better force distribution over the cord, better evacuation of swarf and improved sawing cord life duration.
Recently, novel ways of producing beads are being explored by laser cladding. A first suggestion to make sawing beads by means of laser cladding was published in WO 2002/06553 A2. In there the generic idea of using a laser cladding gun to produce sawing beads is described, but the disclosure gives no instructions as to what materials to use and the finer details of the process and the bead properties obtained. Furthermore the abrasive layer is produced on a solid rod that further has to be turned and drilled in a lathe to obtain the final bead leading to increased cost and handling.
It turns out to be difficult to directly produce an abrasive layer on a tiny metal sleeve with a mass that does not go above 1 gram. In recent publications WO 2012/119946 and WO 2012/119947 the current inventors have endeavoured ways to overcome the heating problems associated with such deposition and found beneficial properties of the bead resulting therefrom. One of the major problems one encounters is the heat balance during the deposition of the abrasive layer on a sleeve with no heat sink. If the input of heat is too high, the sleeve melts and/or the diamonds are damaged. If the input is too low, the coating is porous and/or the process is not economical.
Generally laser cladding with diamond containing coatings is applied on substrates with a large heat sink: massive substrates such as pump rotors, turbine vanes, . . . weighing several kilos wherein excess heat is easily drained towards the interior of the substrate. For example in US 2008/0131620 a process is described for applying a wear resistant coating to an article of manufacture wherein the temperature is kept between 400° C. and 900° C. combined with rapid cooling of the process with the explicit goal to prevent decomposition of the diamond.
Contrary to a wear resistant coating, the abrasive layer of a sawing bead must abrade away at the same pace that diamonds are worn in the sawing process. If the matrix wears too fast, abrasive particles will be dislodged while not being used to the full leading to premature tool failure. Conversely, if the matrix is too wear resistant, abrasive particles will not protrude sufficient for sawing resulting in too slow sawing.
In the following, the improved processes for making sawing beads are revealed together with the properties of the sawing beads resulting therefrom and the sawing cord made therewith.